Process and apparatus for controlling the performance of a homogeneous charge compression ignition (hcci) engine

ABSTRACT

A process for controlling the performance of a homogeneous charge compression (HCCI) engine in a vehicle having a hydrocarbon fuel reservoir which process is provided. The octane or cetane number of hydrocarbon fuel being supplied to the HCCI engine is adjusted by: (a) converting a portion of hydrocarbon fuel from the hydrocarbon fuel reservoir to synthesis gas; (b) converting synthesis gas produced in step (a) to a mixture of hydrocarbons having an octane number less than or a cetane number higher than that of the hydrocarbon fuel of the hydrocarbon fuel reservoir using a Fischer Tropsch process; (c) delivering (i) a portion of hydrocarbon fuel from the hydrocarbon fuel reservoir and (ii) a portion of the mixture of hydrocarbons produced in step (b) to the HCCI engine; and (d) varying the amounts of (i) and (ii) in step (c) in order to adjust the octane or cetane number of the hydrocarbon fuel being supplied to the HCCI engine. Apparatus suitable for carrying out this process is also disclosed.

The present invention relates to a process for controlling theperformance of a HCCI engine comprising adjusting the octane or cetanenumber of a hydrocarbon fuel being delivered to the HCCI engine. Thepresent invention also provides apparatus suitable for use in saidprocess.

Homogeneous Charge Compression Ignition (HCCI) is a distinct combustionmode in which a premixed homogeneous charge of air and fuel iscompressed until it autoignites. Unlike conventional CompressionIgnition (CI) and Spark Ignition (SI) engines, in an HCCI engine, thecombustion reaction initiates at multiple locations simultaneously. AHCCI engine can run on a dilute air and fuel mixture. As a result a HCCIengine operates at a relatively low combustion temperature which leadsto low NO_(x) levels and a level of efficiency normally associated witha CI engine. These advantages make HCCI engines seem an attractivealternative to CI and SI engines.

However, one of the problems associated with HCCI engines is that ofcontrolling ignition timing and combustion over a wide range ofoperating conditions. Factors which influence HCCI ignition andcombustion are temperature, pressure and composition of the fuel and airmixture. The fundamental processes of HCCI combustion make cold startsdifficult without some compensating mechanism. Changing the power output(e.g. by varying the load or speed) of an engine requires a change infuelling rate and therefore a change in charge mixture. This will havean effect on ignition timing and combustion in a HCCI engine and, forexample, at high loads, HCCI combustion can become very rapid andintense and difficult to control. As a result of these difficulties,most engines employing HCCI have dual mode combustion systems in whichtraditional SI or CI combustion is used for operating conditions inwhich HCCI operation is hard to control. Typically, such an engine iscold-started as an SI or CI engine, then switched to HCCI mode for idleand low- to mid-load operation to obtain the benefits of HCCI in thisregime and then switched to SI or CI operation at high load. It ishighly desirable to have improved control over the ignition timing of aHCCI engine in order to extend the load range over which it can beoperated in HCCI mode.

The present invention provides a process for controlling the performanceof a homogeneous charge compression ignition (HCCI) engine in a vehiclehaving a hydrocarbon fuel reservoir which process comprises adjustingthe octane or cetane number of the hydrocarbon fuel being delivered tothe HCCI engine by:

-   (a) converting a portion of hydrocarbon fuel from the fuel reservoir    to synthesis gas;-   (b) converting synthesis gas produced in step (a) to a mixture of    hydrocarbons having an octane number less than or a cetane number    greater than that of the hydrocarbon fuel of the fuel reservoir    using a Fischer Tropsch process;-   (c) delivering (i) a portion of hydrocarbon fuel from the    hydrocarbon fuel reservoir and (ii) a portion of the mixture of    hydrocarbons produced in step (b) to the HCCI engine; and-   (d) varying the amounts of (i) and (ii) in step (c) in order to    adjust the octane number of the hydrocarbon fuel being supplied to    the HCCI engine.

This process provides a way of controlling the ignition timing and thecombustion of a HCCI engine. The process of the invention isparticularly advantageous in that it can be carried out onboard avehicle and in that the vehicle need only be fuelled by a singleconventional hydrocarbon fuel. A portion of the conventional fuel isreprocessed onboard the vehicle and the products of this reprocessingare used to control the overall octane number of the fuel being suppliedto the engine and thereby control the ignition timing.

In this specification the terms “low octane” and “high cetane” aretreated as being identical. Similarly, the terms “high octane” and “lowcetane” are treated as being identical. In this specification, octanenumber means Research Octane Number (RON).

The hydrocarbon fuel may be gasoline or a suitable alternativehydrocarbon fuel. Preferably the hydrocarbon fuel has an octane number(RON) greater than 92.

The HCCI engine is any engine which can work in HCCI mode and may besuitable for use in most vehicles powered by internal combustion engine,particularly light duty vehicles such as passenger cars and smallautomobiles.

The invention will now be described in more detail with reference to,but not limited by, the following figures:

FIG. 1 is a schematic of a configuration for a process according to theinvention.

FIG. 2 is a schematic of a process configuration similar to that of FIG.1 which includes a pressurised container for storing products from thecondensing step of step (b) of the process.

FIG. 3 is a schematic of a process configuration similar to that of FIG.1 which includes a stage for the removal of sulphur-bearing species.

FIG. 4 is a schematic of a process configuration similar to that of FIG.1 which includes a stage for the removal of hydrogen.

The process of the invention may be represented by the configurationshown in FIG. 1. The hydrocarbon fuel is stored in fuel reservoir ortank (1). A portion of the hydrocarbon fuel from the fuel reservoir canbe passed to reformer (6) by means of valve (3). Step (a) of the processof the invention comprises converting hydrocarbon fuel to synthesis gas,i.e. carbon monoxide and hydrogen, also known as syngas. This step iscarried out using a reformer. An example of a suitable reformer is thatdescribed in WO 99/48805. WO 99/48805 discloses compact technology for aprocess for the catalytic generation of hydrogen and carbon oxides whichcombines steam reforming and partial oxidation. In order to perform asteam reforming function, the reformer requires a source (4) of water orsteam. In one embodiment of the present invention, water is obtained bycondensing the exhaust gas stream from the engine or the vehicle. Theaddition of steam or water to the reformer is controlled by valve (5). Asource (15) of oxygen is also required for the manufacture of synthesisgas. In one embodiment, this source of oxygen is an air inlet system.The addition of oxygen and/or air into the reformer is controlled byvalve (7).

Step (b) of the process of the present invention comprises subjectingthe synthesis gas produced in step (a) to a Fischer Tropsch process toproduce a mixture of hydrocarbons having an octane number less than or acetane number greater than that of the hydrocarbon fuel from the fuelreservoir. Preferably, this is followed by a condensing step whichproduces a light gaseous fraction and a heavy fraction. Both the lightfraction and the heavy fraction will have an octane number less than ora cetane number greater than that of the hydrocarbon fuel from the fuelreservoir. Preferably the mixture of hydrocarbons produced by theFischer Tropsch reaction and/or the heavy fraction from the condenserwill have an octane number less than 80 or a cetane number greater than65. The mixture of gases (mainly hydrogen and carbon monoxide) leavingthe reformer are passed to a Fischer Tropsch Unit or reactor (9).Preferably, the hydrogen to carbon monoxide ratio in this mixture is inthe range of 0.6 to 2.5 part hydrogen gas for every part carbonmonoxide. The optimal ratio will depend on the particular FischerTropsch catalyst used. Suitable Fischer Tropsch reactors are describedin Baird et al. (Ind. Eng. Chem. Prod. ResDev 1980 19 175-191). Thereactor may be a fixed bed, fluidized bed or slurry phase reactor. Theprocess may be a high temperature (300-350° C.) Fischer Tropsch processwhich typically utilises iron-based catalysts or a low temperature(200-240° C.) which typically utilises iron- or cobalt-based catalysts.Preferably the products of the Fischer Tropsch reaction (beforecondensing) are gaseous as this simplifies the technology making iteasier to install onboard a vehicle.

As the Fischer Tropsch reaction is exothermic (requiring heat to betaken away), heat needs to be removed from the reaction. For all FischerTropsch catalysts, an increase in temperature results in a shift towardslower carbon products and increases the degree of producing branchedchain isomers. Both of these are undesirable given the objective ofproducing a mixture of hydrocarbons with a low octane number. In apreferred embodiment of the present invention heat from the FischerTropsch reaction of step (b) is utilised by the reforming reaction ofstep (a). This is achieved by thermally integrating the Fischer Tropschreactor and the reformer. The ratio of oxygen gas to carbon and water tocarbon entering the reformer can be used to control whether or not thereforming process is endothermic (requiring an input of heat) orexothermic. By controlling the ratio of water to carbon and the ratio ofoxygen gas to carbon entering the reformer, the carbon monoxide tohydrogen gas ratio of the resultant synthesis gas can be controlled andwhether the net process for reforming will be endothermic or exothermicor balanced in terms of heat production can be determined. In theprocess represented by FIG. 1, these ratios are controlled by valves(3), (5), (7) and (8). During a start up phase of the engine, it isappropriate for the reformer to operate in an exothermic mode.Preferably the reformer then switches to an endothermic mode in steadystate operation and heat is transferred from the Fischer Tropschreaction to the reforming process. In one embodiment of the process ofthe invention heat is transferred from the Fischer Tropsch reaction andused to preheat the reactants in the reformer, i.e. heat is transferredto the reforming process. Even more preferably, heat is transferreddirectly form the Fischer Tropsch reactor to the reformer, although theextent to which this can happen will depend on the relative temperaturesof the Fischer Tropsch reactor and the reformer.

In one embodiment of the invention, an ambient temperature condenser(10) can be used to perform a simple fractionation of the products ofthe Fischer Tropsch reactor. This condensing step produces a lightgaseous fraction and a heavy fraction or condensate. Condensers such asthose described in Boyer and Trumfheller (Chemical Engineering, May 1993issue, McGraw Hill publishing) adapted for an engine are suitable. In apreferred embodiment the cooling water for the condenser is integratedwith the cooling system for the HCCI engine and/or the vehicle. Thecondensate will contain straight chain alkanes with a carbon number ofsix or higher. These will have an octane rating that is lower than thehydrocarbon fuel and/or have a cetane rating that is higher than thehydrocarbon fuel. The injection of condensate into the HCCI engine iscontrolled by valve (11). The lighter gaseous fraction from thecondenser will have an octane rating that is higher than that of thecondensate. If valve (8) is opened and valve (12) is closed, then thelight Fischer Tropsch products are recycled to the reformer forsynthesis gas manufacture. Thus in an embodiment of the process of thepresent invention the light fraction in recycled to the reformer formanufacture of synthesis gas. Alternatively, if valve (12) is opened andvalve (8) is closed then the light Fischer Tropsch products may beinjected directly into the HCCI engine. In step (c) of the process ofthe invention, either the light or the heavy fraction may be deliveredto the HCCI engine.

A portion of the hydrocarbon fuel from the fuel reservoir may beinjected into the HCCI engine (14) by operation of valve (2).

In step (d) of the process of the invention, the overall octane orcetane number of the fuel entering the engine is controlled bycontrolling the amounts of hydrocarbon fuel, light fraction and heavycondensate being delivered to the engine. In the process represented byFIG. 1, these amounts are controlled by valves (2), (11) and (12).

In FIG. 2, the light fraction from the condenser passes through anon-return valve (15). If valve (8) is opened and valves (12) and (16)are closed, then the light Fischer Tropsch products are recycled to thereformer (6) for syngas manufacture. Alternatively, if valve (12) isopen, and valves (8) and (16) are closed, then the light Fischer Tropschproducts are injected directly into the HCCI engine. Alternatively, ifvalve (16) is opened and valve (8) and (12) are closed, then the lighterfraction can be stored in a pressurised storage container (17). The gasstored in (17) can subsequently be injected into the engine by openingvalves (16) and (12) while keeping (8) closed. The stored gas can alsobe recycled for syngas manufacture by closing valve (12) and openingvalves (16) and (8).

In FIG. 3, a trap (20) to remove sulphur and/or sulphur-bearing speciesis introduced between the reformer and the Fischer Tropsch reactor. Thetrap may comprise, for example, zinc oxide pellets. In this embodimentof the present invention, sulphur is removed from the synthesis gasproduct of step (a) before it is passed to the Fischer Tropsch reactorof step (b).

In FIG. 4, hydrogen is selectively removed from the gas stream betweenthe reformer and the Fischer-Tropsch reactor by means of a hydrogenremoval unit (30). This is so as to further control the carbon monoxideto hydrogen ratio for the inlet to the Fischer Tropsch reactor. Thehydrogen removal unit may simply be a palladium membrane. The hydrogenremoved may optionally be used to generate electricity in a ProtonExchange Membrane Fuel Cell (31). This leads to enhanced overallefficiency of a vehicle being powered by the engine as the electricitycould be used to power a number of auxiliary systems of the vehicle, forexample, the air conditioning.

In an embodiment of the present invention, the process for controllingthe performance of a HCCI engine comprises a step of monitoring theengine's performance and adjusting the octane or cetane number of thefuel being delivered to the HCCI engine accordingly. Preferably a sensoris provided in the engine and this is used to monitor the engine'sperformance. Preferably the sensor is a knock sensor. Preferably theengine comprises a engine management chip which receives informationfrom the sensor and processes said information to provide data fromwhich the required adjustment to the octane/cetane number of the fuel,i.e. the amounts of (i) and (ii) in step (c) of the process of theinvention, can be determined.

The present invention also provides a HCCI engine and fuel systemsuitable for use in the above-described process comprising

-   (a) a fuel reservoir;-   (b) a HCCI engine;-   (c) a reformer for converting hydrocarbon fuel to synthesis gas;-   (d) a reactor for converting synthesis gas to a hydrocarbon mixture    using a Fischer Tropsch process;-   (e) means for removing a portion of hydrocarbon fuel from the fuel    reservoir and delivering it to the reformer;-   (f) means for removing synthesis gas product from the reformer and    delivering it to the reactor;-   (g) means for removing a mixture of hydrocarbons from the Fischer    Tropsch reactor and delivering it to the HCCI engine; and-   (h) means for removing a portion of hydrocarbon fuel from the fuel    reservoir and delivering it to the HCCI engine.

Preferably the HCCI engine and fuel system further comprises (i) acondenser for converting a hydrocarbon mixture produced by the FischerTropsch reactor into a light and a heavy fraction and the meansspecified in (g) includes means for removing a portion of a heavyfraction and/or a light fraction produced by the condenser anddelivering it to the HCCI engine.

Means (e) to (i) may comprise a system of valves.

In one embodiment, the HCCI engine and fuel system further comprises asensor for monitoring the engine's performance. Preferably this sensoris a knock sensor. Preferably this embodiment further comprises anengine management chip.

In a further embodiment of the invention, the HCCI engine and fuelsystem comprises one or more of:

-   (a) a pressurised storage container for storing the light fraction    produced by the condenser;-   (b) a trap for removing sulphur from the synthesis gas being fed    into the Fischer Tropsch reactor; and-   (c) a hydrogen removal unit for removing hydrogen from the synthesis    gas product from the reformer before the synthesis gas is fed to the    Fischer Tropsch reactor.

The present invention also provides a vehicle comprising theabove-described engine and fuel system.

1. A process for controlling the performance of a homogeneous charge compression (HCCI) engine in a vehicle having a hydrocarbon fuel reservoir comprising: (a) converting a portion of hydrocarbon fuel from the hydrocarbon fuel reservoir to synthesis gas; (b) converting synthesis gas produced in step (a) to a mixture of hydrocarbons having an octane number less than or a cetane number higher than that of the hydrocarbon fuel of the hydrocarbon fuel reservoir using a Fischer Tropsch process; (c) delivering (i) a portion of hydrocarbon fuel from the hydrocarbon fuel reservoir and (ii) a portion of the mixture of hydrocarbons produced in step (b) to the HCCI engine; and (d) varying the amounts of (i) and (ii) in step (c) in order to adjust the octane or cetane number of the hydrocarbon fuel being supplied to the HCCI engine, thereby adjusting the octane or cetane number of hydrocarbon fuel being supplied to the HCCI engine.
 2. The process of claim 1 wherein step (a) comprises steam reforming using a source of water or steam, at least part of the said water or steam required for step (a) is obtained by condensing an exhaust gas stream from the engine or vehicle.
 3. The process of claim 1 wherein the synthesis gas used in step (b) has a hydrogen gas to carbon monoxide ratio in the range of 0.6 to 2.5 part hydrogen gas for every part carbon monoxide.
 4. The process of claim 1 wherein heat of reaction from the Fischer Tropsch process of step (b) is used to: (i) provide heat for the reaction of step (a); and/or (ii) preheat the reactants for the reaction of step (a).
 5. The process of claim 1 wherein the Fischer Tropsch process in step (b) is followed by a condensing step which produces a light and a heavy fraction and wherein the light and/or the heavy fraction form at least part of the mixture of hydrocarbons in (ii) of step (c).
 6. The process of claim 5 wherein the condensing step produces a light fraction and a heavy fraction and at least a part of the light fraction is either (a) stored in a pressurised storage container; (b) delivered to the HCCI engine; or (c) recycled by converting it to synthesis gas in step (a).
 7. The process claim 5 wherein cooling water for the condensing step is provided by a cooling system for the HCCI engine or vehicle.
 8. The process of claim 1 wherein the process further comprises (e) monitoring the performance of the HCCI engine using a sensor to provide information relating to engine performance and adjusting the octane number or cetane number in response to said information.
 9. The process of claim 8 wherein the sensor is a knock sensor.
 10. The process of claim 8 wherein an engine management chip receives information from the sensor and processes said information to provide data from which the required amounts of (i) and (ii) of step (c) can be determined.
 11. A HCCI engine and fuel system comprising (a) a fuel reservoir; (b) a HCCI engine; (c) a reformer for converting hydrocarbon fuel to synthesis gas; (d) a reactor for converting synthesis gas to a hydrocarbon mixture using a Fischer Tropsch process; (e) means for removing a portion of hydrocarbon fuel from the fuel reservoir and delivering it to the reformer; (f) means for removing synthesis gas product from the reformer and delivering it to the reactor; (g) means for removing a mixture of hydrocarbons from the Fischer Tropsch reactor and delivering it to the HCCI engine; and (h) means for removing a portion of hydrocarbon fuel from the fuel reservoir and delivering it to the HCCI engine.
 12. The HCCI engine and fuel system of claim 11 which further comprises (i) a condenser for converting a hydrocarbon mixture produced by the reactor into a light and a heavy fraction; and the means specified in (g) includes means for removing a portion of the heavy fraction and/or the light fraction produced by the condenser and delivering it to the HCCI engine.
 13. The HCCI engine and fuel system of claim 11 or comprising (k) a sensor for monitoring the engine's performance.
 14. The HCCI engine and fuel system of claim 13 comprising (1) an engine management chip.
 15. The HCCI engine and fuel system of claim 11 which further comprises one or more of: (a) a pressurised storage container for storing the light fraction produced by the condenser; (b) a trap for removing sulphur from the synthesis gas being fed into the Fischer Tropsch reactor; and (c) a hydrogen removal unit for removing hydrogen from the synthesis gas product from the reformer before the synthesis gas is fed to the Fischer Tropsch reactor.
 16. A vehicle comprising the HCCI and fuel system of claim
 10. 17. A vehicle comprising the HCCI and fuel system of claim
 15. 18. The process claim 6 wherein cooling water for the condensing step is provided by a cooling system for the HCCI engine or vehicle.
 19. The process of claim 5 wherein the process further comprises (e) monitoring the performance of the HCCI engine using a sensor to provide information relating to engine performance and adjusting the octane number or cetane number in response to said information.
 20. The process of claim 9 wherein an engine management chip receives information from the sensor and processes said information to provide data from which the required amounts of (i) and (ii) of step (c) can be determined. 